In a semiconductor fabrication process, tiny electronic devices are mass manufactured on semiconductor wafers, which are thin slices of crystalline silicon ranging anywhere from 3 inches to 8 inches in diameter. The devices are so small that contact by dust particles can render them defective. Cleanliness is therefore an absolute necessity for profitable semiconductor fabrication.
Because many different types of chemicals and process steps are required to manufacture such devices, wafers must also be thoroughly cleaned between each step so that residual chemicals from one step do not interfere with the chemicals of the next step and render product defective.
Wafers are periodically cleaned during the process to remove particles and residual chemicals by washing them with de-ionized (DI) water. Simple washing is not enough--how do you know if the wash was effective? One way is to examine the discharge water from the washing process. If a wafer is dirty, the discharge water will be contaminated with particles and chemicals. If a wafer is clean, the discharge water will be pure.
Pure DI water has a high resisitivity, which means that it tends not to conduct electricity. Dirty DI water has a lower resistivity: the contaminants in the water provide a conductive path for electricity. Measuring resistivity of the discharge water is therefore one way to verify wafer wash effectiveness.
Discharges from successive washes will show resistivities that start low and steadily increase as the discharges contain less and less contaminants. An empirical resistivity threshold value can then be determined that indicates a clean wafer and a final wash cycle.
A machine called a wafer washer performs such washings. The washer rapidly spins one or more wafers while spraying DI water (to wash) or dry nitrogen (to dry) on the wafers. Discharge water exits the washer and splashes by a resistivity probe, which is generally mounted on or near the washer discharge tube. The resistivity probe then provides a signal which indicates the purity of the discharge water and hence the cleanness of the wafers being washed.
The resistivity probe arrangement of one commonly used wafer washer needs improvement. This configuration is shown in FIG. 1. A discharge tube 101 exits into an open-topped discharge chamber 102. Discharge water splashes out of the tube 101 into the chamber 102, contacting a resistivity probe 103 and finally exiting through an exit hole 104. The chamber is attached to the back of the washer, generally enclosed in a plenum. This configuration, although adequate, has some disadvantages.
One disadvantage is the chamber construction. Because it is closely attached to the washer housing, chamber maintenance is difficult. The chamber is not easily replaceable or easily cleanable. An easily replaceable, even disposable (in the event of massive contamination) chamber would therefore be advantageous.
Another disadvantage is the substantially open configuration of the chamber, which exposes the resistivity probe to drafting plenum air, which is not optimally clean. A dirty probe will give false readings. Another problem with the open configuration is the tendency of contaminated water to splash out. If the contamination is hazardous and someone is near, a safety danger arises, as well as a contamination risk from the splashing water. A substantially closed chamber configuration would therefore be an improvement.
Another disadvantage is the splashy contact of the discharge water with the probe. A consistent and accurate resistivity reading is difficult when the water merely splashes by. The probe measures through water at one instant, and through air at another instant, so a false reading can be taken, the result being that a wafer is not washed thoroughly, risking subsequent yield loss. Therefore, finding a way to fully submerge the probe in the discharge water would be advantageous.